The present invention relates generally to integrated circuits and methods of manufacturing integrated circuits. More particularly, the present invention relates to generating phase shifting patterns to improve the patterning of gates and other layers, structures, or regions needing sub-nominal dimensions.
Semiconductor devices or integrated circuits (ICs) can include millions of devices, such as, transistors. Ultra-large scale integrated (ULSI) circuits can include complementary metal oxide semiconductor (CMOS) field effect transistors (FET). Despite the ability of conventional systems and processes to fabricate millions of IC devices on an IC, there is still a need to decrease the size of IC device features, and, thus, increase the number of devices on an IC.
One limitation to achieving smaller sizes of IC device features is the capability of conventional lithography. Lithography is the process by which a pattern or image is transferred from one medium to another. Conventional IC lithography uses ultra-violet (UV) sensitive photoresist. Ultra-violet light is projected to the photoresist through a reticle or mask to create device patterns on an IC. Conventional IC lithographic processes are limited in their ability to print small features, such as contacts, trenches, polysilicon lines or gate structures.
Generally, conventional lithographic processes (e.g., projection lithography and EUV lithography) do not have sufficient resolution and accuracy to consistently fabricate small features of minimum size. Resolution can be adversely impacted by a number of phenomena including: diffraction of light, lens aberrations, mechanical stability, contamination, optical properties of resist material, resist contrast, resist swelling, thermal flow of resist, etc. As such, the critical dimensions of contacts, trenches, gates, and, thus, IC devices, are limited in how small they can be.
For example, at integrated circuit design feature sizes of 0.5 microns or less, the best resolution for optical lithography technique requires a maximum obtainable numerical aperture (NA) of the lens systems. Superior focus cannot be obtained when good resolution is obtained and vice versa because the depth of field of the lens system is inversely proportional to the NA and the surface of the integrated circuit cannot be optically flat. Consequently, as the minimum realizable dimension is reduced in manufacturing processes for semiconductors, the limits of conventional optical lithography technology are being reached. In particular, as the minimum dimension approaches 0.1 microns, traditional optical lithography techniques may not work effectively.
With the desire of reducing feature size, integrated circuit (IC) manufacturers established a technique called xe2x80x9cphase shifting.xe2x80x9d In phase shifting, destructive interference caused by two adjacent translucent areas in an optical lithography mask is used to create an unexposed area on the photoresist layer. Phase shifting exploits a phenomenon in which light passing through translucent regions on a mask exhibits a wave characteristic such that the phase of the light exiting from the mask material is a function of the distance the light travels through the mask material. This distance is equal to the thickness of the mask material.
Phase shifting allows for an enhancement of the quality of the image produced by a mask. A desired unexposed area on the photoresist layer can be produced through the interference of light from adjacent translucent areas having the property that the phase of the light passing through adjacent apertures is shifted by 180 degrees relative to each other. A dark, unexposed area will be formed on the photoresist layer along the boundary of the phase shifted areas caused by the destructive interference of the light which passes through them.
Phase shifting masks are well known and have been employed in various configurations as set out by B. J. Lin in the article, xe2x80x9cPhase-Shifting Masks Gain an Edge,xe2x80x9d Circuits and Devices, March 1993, pp. 28-35. The configuration described above has been called alternating phase shift masking (PSM).
In some cases, phase shifting algorithms employed to design phase shifting masks define a phase shifting area that extends just beyond active regions of an active layer. The remaining length of polysilicon, for example, is typically defined by a field or trim mask. However, this approach is not without its problems. For example, alignment offsets between phase shift masks and field masks may result in kinks or pinched regions in the polysilicon lines as they transition from the phase shifting area to the field mask areas. Also, since the field masks are employed to print the dense, narrow lines of polysilicon beyond the active regions, the field masks become as critical and exacting as the phase shift masks.
Phase shift patterning of polysilicon or xe2x80x9cpolyxe2x80x9d layouts has been proven to be an enhancement in both manufacturing as well as enabling smaller patterned lines and narrow pitches. These items can be more enhanced as the desired linewidth and pitch shrinks, yet there can be some risks and complications.
Conventional patterning with phase shifters has been done by shifting only the areas of minimum desired dimensionsxe2x80x94usually the poly gate or narrow poly that is over the active pattern. The patterned poly lines that are away from the active regions are usually laid out with similar design rules as that of the patterned poly lines on active regions. As such, there can be many transitions between the phase shifted patterning and binary patterning. Transition areas can result in linewidth loss, increasing device leakage.
Current alternating phase shift masking (PSM) designs for polysilicon layers often focus on enabling gate shrink by applying alternating phase shift regions around the gate region (i.e., the intersection of the polysilicon and active layers). One such alternating PSM design is described in U.S. Pat. No. 5,573,890 entitled METHOD OF OPTICAL LITHOGRAPHY USING PHASE SHIFT MASKING, by Christopher A. Spence (one of the inventors of the present application) and assigned to the assignee of the present application.
An enhanced phase shift approach was developed to reduce the transition regions and move those regions away from the active edge to wider poly or corners of poly patterns where linewidth loss would have little or no impact. Examples of this enhanced phase shifting approach are described in U.S. patent application Ser. No. 09/772,577, entitled PHASE SHIFT MASK AND SYSTEM AND METHOD FOR MAKING SAME, filed on Jan. 30, 2001, by Todd P. Lukanc (one of the inventors of the present application) and assigned to the assignee of the present application, incorporated herein by reference.
The specification of the Lukanc patent application describes binary and phase masks that define parts of the poly pattern and need to have very controlled critical dimensions (CDs). The phase mask basically has long narrow openings that are easy to pattern but the binary mask has both small openings as well as small lines, in both isolated and dense areas. As such, the patterning of the binary mask can be complicated and the manufacturing window of this technique can be limited. In both the simple phase and the enhanced phase methods, both masks are critical and have different optimized illumination and patterning conditions.
Other known systems use a xe2x80x9cnodexe2x80x9d based approach rather than a gate-specific approach to generate a phase assignment that attempts to apply phase shifting to all minimum poly geometries (both field and gate). Two examples of the xe2x80x9cnodexe2x80x9d based approach include, for example, Galan et al. xe2x80x9cApplications of Alternating-Type Phase Shift Mask to Polysilicon Level for Random Logic Circuits,xe2x80x9d Jpn. J. Appl. Phys. Vol. 33 (1994) pp. 6779-6784, December 1994, and U.S. Pat. No. 5,807,649 entitled LITHOGRAPHIC PATTERNING METHOD AND MASK SET THEREFOR WITH LIGHT FIELD TRIM MASK, by Liebmann et al.
In view of the known art, there is a need for improvements to the clear field phase shifting mask (PSM) and field or trim mask approach that result in simpler and more reliable mask fabrication and in better wafer imaging. Further, there is a need to minimize variations or use of optical proximity correction (OPC) by enclosing phase shift masking features. Yet further, there is a need to generate phase shifting patterns to improve the patterning of gates and other layers needing sub-nominal dimensions.
An exemplary embodiment relates to a method of patterning gates to increase process margins from conventional methods. This technique can be called Full Phase patterning and can define all poly patterns with a phase mask, using only a field or trim mask to resolve conflicts in the phase mask. The trim mask can expose a series of lines that either separates the different phase areas where patterns are not desired or minimizes the range of sizes of the phase patterns next to a critical gate area.
An exemplary embodiment of Full Phase pattern generation can be carried out as follows. First the critical poly regions or other region desired to be defined by phase shifting and the poly adjoining are defined. Phase regions are then created on either side of these critical poly regions. These phase regions are assigned phase angles such that regions on either side of the critical poly are 180 degrees out of phase. A next step is to define all remaining poly edges and join as much of these edges to the phase assigned areas as possible. The phase transitions are kept at wide poly areas or at corners or line ends. Next, a boundary around the phase 180 edges not defining the poly edge is defined, and then all area outside that boundary is defined as phase 0. Subnominal lines, spaces, and other violation areas can be cured and optical proximity correction (OPC) can be applied to appropriate areas. Two masks are created. The first is a substantially clear-field phase mask that fully defines the poly pattern and additional artifacts, such as, phase conflicts and phase breaks. The second, a trim mask, is a substantially dark-field mask which removes the phase conflict and phase break regions without impacting the phase-defined poly pattern. The critical patterns on both masks are narrow openings and, thus, have almost identical optimized illumination and patterning conditions.
Another exemplary embodiment is related to a method of defining a phase shifting mask. This method can include defining critical poly regions and adjoining poly where the critical poly regions are regions desired to be defined by phase shifting, creating phase regions on either side of the critical poly regions, assigning phase angles to the phase regions such that the phase regions have either a first phase angle or a second phase angle, defining edges of the phase regions being assigned the second phase angle where the edges do not define a poly pattern, defining a boundary region around the defined edges, and defining regions outside a desired poly pattern, phase regions, and boundary region to have the first phase angle. The desired poly pattern, phase regions, and boundary region define a mask.
Another exemplary embodiment is related to a method of generating phase shifting pattern to improve the patterning of gates and other layers needing sub-nominal dimensions. This method can include defining critical areas, creating phase regions on either side of these critical areas, assigning opposite phase polarities to each side of the critical areas, enhancing the area of the phase regions to reduce patterning errors, defining break regions where phase transitions will be easy to remove later with the trim mask, generating polygons to define other edges and excluding the defined break regions, merging the generated polygons with enhanced critical areas having a common phase polarity, separating the polygons having interactions with more than one polarity into portions which are merged into regions having only one polarity, constructing a boundary region outside of phase 180 regions, and defining undefined regions as phase zero regions.
Another exemplary embodiment is related to a method of enhancing clear field phase shift masks with a border around outside edges. This method can include assigning phase polarities to the phase regions, defining the edges of the phase regions not defining the poly, establishing a boundary region around these defined edges, and assigning outside of the established boundary to have phase zero.
Other principle features and advantages of the invention will become apparent to those skilled in the art upon review of the following drawings, the detailed description, and the appended claims.